Potentiometer compensating machine



Oct. 27, 1953 H. R. BROWN, JR

POTENTIOMETER COMPENSATING MACHINE Filed Nov. 24, 1951 FIG.

INVENTOR.

HENRY R. BROWN JR.

War w ATTORNEY Patented Oct. 27, 1953 UNITED OFFICE POTENTIOMETER COMPEN'SKTING MHCHINE' Henry R'rBrcw-n, Jr.-,,Dwney,;Calif., assignon-to North American Aviation, Inc.

ApplicationNovembcr'zl, 195'1', SerialNo. 2585083 11 Glaims.. (Clr 201-62) which isgrounded; a voltage proportional tothe product X6 is. obtained fromthe W-iper oi the second potentiometer if the shaft of the second potentiometer is turned through anangle 7. If thevoltage from the-Wiper of the second poten tiometer is then fed through a second load resistor to a fixed-terminalofa third potentiometer, theother fixecl terminal of which is grounded; a voltage proportional to the-product X-Zi w is developed-on-thewiper of the third=potentiometer if the shaft of the third potentiometer is turned through anangle a: Inord'er that the resultant product may beformed with some precision, the loadresistors" for the second twopotent-iome'ter's must be at least two-orders of magnitude larger than the total resistance of each of thep0ten--- tiometers. The result, therefore; isthat-though the voltage may be fairlylarge, the voltage Xtva will"v be: exceedingly smail on accountof the size: oi the intervening resistors.- Further, the load resistors provide a-parallel path toground which seriously affects the accuracy of cascaded potentiometers as; multiplying devices.

If. it: is required that a potentiometer, having applied to" one of it'sfixed terminals a given fixed" voltage, must supply as the output of its wiper a voltage which isa given function of the potentiometer shaft rotation, it can be seen that the simple utilization of a. potentiometer Whose resistance vs. shaft rotation: function is, the samej as'the: voltagevsi-shaftrrotation function required tentiometer, butv must be computed? by reference to Kirchoffs lawof currents since the wiper doesdraw current. In addition,.while,potentiometers have heretoforez'been available which would pro."-

This is true because thoughthe vide a: given function-oi resistance vs. shaftrotatlonwith-in limits of a few per-cent,.sincethetolerano'e' could not be entirely eliminated, it has been necessary to-adjust the resistancevs. shaiit rota.-

ti'on characteristics 01 each potentiometer before it could be inserted inacircuit toattain the desired l precise result. Inaddition, tolerances exist" upon the total resistance 01 a potentiometer which therefore necessitated the adjustment. of the total resistance or a potentiometer berore' it could beutilized in a precision computer circuit.

This invention contemplates providing a device for automatically compensating a potentiometer of the type described inpatent application Serial No. 2553898, filedNovemper-lS, 1951, in the name 0t Henry Brown, Jr .,-i'oi Compensated Mini-- ature- Potcntiometer and Method of Makingthe compensation being applied in such a: mannor as to provide not only the exacttotal resi'stance of a potentiometer requirembut also to It is therefore an object of this: invention to provide a potentiometer compensatingdevice adapted to compensate a potentiometer to provide a predetermined resistance vs. shaft rotation function;

It is-another'ohject of this' invention to provide a potentiometer compensating device adapted to compensate a potentiometer to have aprecise predetermined total resistance.

It is an'other object of this invention toprov-ide meansfor compensating'a potentiometer so that the potentiometer will produce a voltage which is-a predetermined" function of shaft rotation with a'predeterminedioadl Other objects of" invention will become apparentfrom' the following description taken in connection with the accompanying drawings, in

which Fig. l is aschematic of theinvention; and

Figrzis a detailed'viewof a part of the i'nven scribed in patent application Serial No. 255,898 o referred to above; which is required tob'e com- 5 pensatedl One fixed terminal of element I is connected to'a fixed terminal of resistor 2'. The

resistance of resistor 2 is'equal to the desired total value of resistance 5 A- wiper 3 supported by n carriage 2-1 contacts the central portion of resistance element landfis advanced along'element I at a constant slow speed by lead screw 28 connected to motor t which also drives the wiper of potentiometer 5. One fixed terminal of potentiometer is supplied with a constant voltage D, and the other fixed terminal is grounded.

Wiper 3 is electrically connected to one fixed terminal of load resistance 6, whose other fixed terminal is grounded. Resistance is is shown as a variable resistance. It is initially set equal to the value of load resistance for which element i is desired to be compensated, and during any one compensating operation, is left unaltered. Resistance 6 simulates the load resistance which will be connected to element I when it is placed in operation. Wiper 3 is also connected to precision inverting amplifier 1 whose characteristic is that its output is equal to its input in magnitude, but is of opposite polarity. The output of amplifier d is connected through resistances i8 and 9 to wiper ll of potentiometer 5. The common connection between resistances 8 and 9 is connected to the input of servo amplifier it which provides electrical drive for servo motor ll. Servo motor II drives lead screw it, producing lateral motion of stylus [2 which cuts into the conducting film on the outer portion of element I. As carriage 2!, shown in detail in Fig. 2, is moved along the length of element 2 exactly in coincidence with wiper 3 by drive motor 3, the lateral motion of the stylus causes the area of conducting film to vary from bar to bar or" element l and effects the compensation, as will be further explained hereinafter.

The output of inverting amplifier l is also connected to Wiper It or" resistance 2. Wiper I3 is ..-'i"

driven mechanically by servomotor it, which is driven electrically by amplifier i l. The input to amplifier Hi is connected to the common connection between the fixed terminals of resistances i and 2. Resistance IE is identical in resistance value with resistance 2, and is thus the same as the desired total resistance of element i. Wiper l5 of resistance It is driven, together with wiper I8, by motor I3, and is connected electrically to wiper 3 of resistance element l. One fixed terminal of resistance i6 is connected to constant voltage D.

At the start of operation, wiper} of resistance element I, and wiper i! of potentiometer 5 will be at their lowermost positions in the figure,

and wiper l5 of resistance It and wiper I8 of resistance 2 will be at their uppermost positions. When motor 4 is started, wipers 3 and I! will be moved upwards. Current fiowing from voltage source D through resistance [6 and resistance 6 to ground will cause a voltage to appear across resistance 6, and hence at wiper 3 and the input to amplifier I. This voltage will be inverted by amplifier l and applied through wiper 18 to the input of amplifier Hi. The action of amplifier i4 and motor I3 is to drive wiper l8 of resistance 2 so that the resistance between wiper l8 and the input of amplifier It is the same as the resistance between wiper 3 of resistance element I and the input of amplifier Hi. When this is the case, the voltage at the input to amplifier I4 will be zero, for this point is then the junction between two equal resistances, the opposite ends of which are connected to voltages equal in magnitude but opposite in polarity.

Since this is a continuous process, motor I3 must cause wiper E8 to move continuously, and there must therefore always be some small voltage at the input to amplifier I l; for with absolutely zero input there will be no output and no 4 voltage to drive motor i3. However, the system can be made sensitive enough so that the voltage input to amplifier M will be extremely small and the error thus introduced will be of negligible magnitude.

Motor I3 also drives wiper l5 of resistance It in a downward direction, and since the total value of resistance I6 is the desired total resistance of resistance element l, the amount of resistance it which is in the circuit at any time is equal to this total desired resistance less the amount between wiper 3 and the lower fixed terminal of resistance element I. This follows from the facts that wiper I5 and Wiper 18 are driven together; resistances i6 and 2 are identical; and the amount or" resistance 2 which is in the circuit is equal to the amount of resistance element I which is in the circuit, neglecting the error mentioned above.

The result of the above is that the voltage apearing at wiper 3 is the same that would appear at that point were resistance element I being used in an actual installation. It is required that resistance element i develop the same function or" voltage vs. shaft rotation as is developed by potentiometer If resistance element I is correct up to the position occupied by wiper 3, since wiper it has been advanced to an exactly esponding position, then the voltage at wiper 3 with load resistance 8 in the circuit would be equal to the voltage at Wiper IT. The voltage at wiper ii is applied to resistance 8. The voltage at wiper if is applied to resistance 9, which has identically the same value as resistance 8. With the voltages at wipers 3 and Il equal, no voltage would appear at the junction of resistances 8 and ii, and hence at the input to amplifier I0. If, however, as would generally be the case, resistance element 5 were not exactly correct, the voltage at wiper 3 would not equal that at wiper l1, and a non-Zero signa1 would appear at the input to amplifier It, driving motor II driving stylus If: to compensate resistance element I. This process of compensation continues as long as motor i is turned, with the result that the voltage developed by wiper 3, as modified by the presence of load resistor is exactly the same function of shaft rotation as is the voltage developed by wiper ll. The combined value of the portions of element 1 and resistance I6, which are in the circuit, is at all times equal to the total desired resistance of element l. The compensating operation reaches an end when all of resistance 2 and of element I is in the circuit and none of resistance :6 is in the circuit. Hence the resultant total value of resistance element I will be equal to the resistance of 2 or of IS, both of wfhlich are equal to the desired total resistance 0 Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

I claim:

1. Means for automatically compensating for function of resistance versus displacement, a load resistance simulating the load resistance to be used in connection with the operation of the potentiometer of which said element is to be a part, means for moving said cutting means along said element in synchronism with said master potentiometer, and servo means for varying said cutting means in response to the difference between the resistance of said element at the instantaneous location of said cutting means as qualified by said load resistance and said master potentiometer to thereby compensate said element.

2. Means for automatically compensating the resistance element of a conducting bar and resistance film type potentiometer, comprising trimming means for rendering noneffective a controllable portion of the resistance of said element, a contact-making wiper on said element, a master potentiometer, means for moving said wiper and said trimming means together and in synchronism with the movement of the wiper on said master potentiometer, a load resistor connected to modify the effective resistance of said element as sampled by said wiper, and means for controlling the amount of trim accomplished by said trimming means in response to the difference between the resistance sampled by the wiper on said master potentiometer and the wiper on said element to thereby compensate said potentiometer element.

3. Means for adjusting the resistance per unit length of a potentiometer resistance element whose unit linear resistance is a function of its lateral dimension, comprising cutting means for varying the lateral dimension of said element, a motor, means for producing a reference voltage which is a predetermined function of the rotation of said motor, a wiper on said element, means for advancing said wiper along said element in synchronism with said motor, means for furnishing to one end of said element a constant direct current voltage, an inverting amplifier with its input taken from said wiper, and means for driving said cutting means in response to the difference between the output of said inverting amplifier and said reference voltage to thereby adjust the resistance per unit length of said potentiometer resistance element to obey said predetermined function.

4. A device as recited in claim 3 and further comprising a predetermined load resistance for qualifying the input to said inverting amplifier whereby said element is compensated for load resistance in the circuit in which it is intended to be used.

5. A device as recited in claim 3 and further comprising means for indicating when the total resistance of said element has reached a predetermined value.

6. Potentiometer compensating means comprising means for adjusting the unit resistance of said potentiometer, a source of reference voltage which varies predeterminately with a shaft rotation, means for supplying to a fixed terminal of said potentiometer a constant direction current voltage, means for moving the wiper on said potentiometer in coordination with said shaft rotation, and means for operating said adjusting 6 means in response to the difference between the voltage on said wiper and said reference voltage to thereby compensate said potentiometer predeterminately.

7. A device as recited in claim 6 and further comprising means for qualifying the operation of said adjusting means in response to the effect of a predetermined load resistance applied to said potentiometer.

8. Apparatus for compensating a film potentiometer of the type whose resistance per unit displacement may be adjusted by varying the effective area of resistive film on the resistance element thereof, comprising a rotatable shaft, means for producing a reference voltage which is a predetermined function of the rotation of said shaft, means for moving the wiper on said potentiometer in response to the rotation of said shaft, means associated with said potentiometer for producing a voltage proportional to the resistance between a fixed terminal of said poten= tiometer and the Wiper of said potentiometer, and means for varying the eifective area of resistive film on the resistance element of said potentiometer in response to the difierence between said reference voltage and the voltage from said potentiometer to thereby compensate said potentiometer in accordance with said predetermined function.

9. A device as recited in claim 8 and further comprising a resistance simulating the load resistance to be used with said potentiometer connected to the wiper of said potentiometer to thereby compensate said potentiometer for the efiect of said load resistance.

10. Means for trimming a film type potentiometer to have a predetermined resistance versus displacement function comprising means for applying a constant voltage to one terminal of said potentiometer, means for producing a reference voltage which is a predetermined function of the displacement of the wiper of said potentiometer, and means responsive to the difference between the voltage on said wiper and said reference for trimming the resistance element of said poten tiometer to thereb cause said potentiometer to yield a resistance which is said predetermined function of the displacement of its wiper.

11. Apparatus for compensating a voltage divider to yield a voltage which is a predetermined function of the displacement of its wiper with current being drawn from said wiper by a load resistance comprising a source of reference voltage variable in accordance with said predetermined function of said displacement and means for compensating said voltage divider in response to the difference between the voltage on the wiper thereof and said reference voltage to thereby compensate said voltage divider.

HENRY R. BROWN, JR.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,859,930 Miller May 24, 1932 1,962,438 Flanzer et al June 12, 1934 2,500,605 De Lange et a1 Mar. 14, 1950 

